This invention relates to gate drive circuits for MOSFETs (Metal Oxide Silicon Field Effect Transistors), and in particular, to methods of switching MOSFETs in power converters. This invention also relates to clamping circuits used to control the overshoot due to the current flowing in the circuit inductances at the turn off of the MOSFETs.
In power converters, it is important to minimize losses overall, and it is particularly important to minimize the losses in the MOSFETs. The gate characteristics have been studied extensively, and the “Miller effect” is well known to anyone who has worked with gate drive circuits. The Miller effect increases the apparent capacitance of the gate to source capacitance, thus require a robust gate drive. Further, during the time that the Miller effect is present, the crossover power dissipation in the MOSFET being switched is very high.
Determining the “Miller current” is fairly involved, but simplified, on turn off, the gate voltage will decrease as the gate capacitance is discharged until the gate voltage reaches the level that sustains the drain current. At this point, the “Miller shelf” becomes evident, that is, the gate voltage remains constant at the “Miller voltage”, and the current out of the gate is determined by the impedance of the gate drive circuit and the Miller voltage.
There is an equal and opposite current into the gate, internal to the MOSFET, through the drain-gate capacitance, and the drain voltage rises at a rate such that the “Miller current” through the drain gate capacitance is in equilibrium with the current out of the gate. Once the drain voltage has reached its upper limit, the Miller current stops flowing through the drain gate capacitance to the gate. At this point, the gate voltage once again decreases, and the drain current decreases accordingly until the gate voltage reaches the cutoff threshold and the MOSFET is turned off.
During most of the turn off sequence, while the MOSFET is in its active region, there is both voltage across and current through the MOSFET drain to source, so there is power dissipated. This is the familiar “crossover power”. It is well known to reduce the crossover power by using a lower impedance gate drive.
As is well known to one skilled in the art, a corresponding Miller effect may occur when the MOSFET is turned on. An exception is with “zero volt” switching.